(a) Field of the Invention
The present invention relates to a method for preparing a nuclear-substituted salicylic acid or a metal salt thereof. More particularly, the present invention relates to a method for preparing a nuclear-substituted salicylic acid or a metal salt thereof from an alkali metal salt of a specific nuclear-substituted phenol by a Kolbe-Schmitt reaction. The nuclear-substituted salicylic acid or the metal salt thereof may be used in wide variety of fields such as antibacterial agents, additives for lubricating oils and plastics and color developing agents-for pressure-sensitive recording sheets.
(b) Description of the Prior Art
Specific nuclear-substituted salicylic acids or metal salts thereof are useful as, in particular, color developing agents for pressure-sensitive recording sheets or materials for preparing the same. Several methods for preparing these compounds have already been proposed. Among these, those making use of Kolbe-Schmitt reactions which are also concerned with the present invention are disclosed in, for instance, Japanese Examined Patent Publication (hereinafter referred to as "J. P. KOKOKU") No. Sho 51-25174, Japanese Un-examined Patent Publication (hereinafter referred to as "J. P. KOKAI") Nos. Hei 1-145189, Hei 2-85231, Hei 2-122977, Hei 2-142747 and Hei 2-204472. However, these patents never recognize the difference in the reaction rates or the reaction yields of the Kolbe-Schmitt reactions, to which the present invention particularly relates, between kinds of alkali metals and, in fact, sodium is exclusively selected and used simply for economical reason.
When nuclear-substituted salicylic acids or metal salts thereof are prepared by the use of Kolbe-Schmitt reactions, the reaction product inevitably includes the unreacted nuclear-substituted phenol in greater or lesser degree. Therefore, if highly pure nuclear-substituted salicylic acids or metal salts thereof are required, the unreacted starting materials as principal impurities are in general removed from the reaction product through a recrystallization or extraction method for the purification of the product.
However, when the nuclear-substituted salicylic acids having specific substituents or metal salts thereof are prepared according to Kolbe-Schmitt reactions, the reaction product includes a relatively large amount of the unreacted starting materials. Moreover, the nuclear-substituted salicylic acids or metal salts thereof are in amorphous forms and cannot be purified through recrystallization. Further, they are highly soluble in extraction solvents and have high ability of emulsifying and dispersing aqueous solutions and, therefore, cannot be purified through extraction. Thus, only the reaction product having a low purity could be obtained.
On the other hand, the nuclear-substituted salicylic acids or metal salts thereof prepared according to the method of this invention are used, in particular, in color developing agents for pressure-sensitive recording sheets. However, those prepared by the conventional Kolbe-Schmitt reactions often have low quality and purities and these products as such cannot be satisfactorily used in this application.